(1) Field of the Invention
This invention relates to a method and apparatus for testing an array of detectors and more particularly to a method and apparatus for testing individual ones of a plurality of hydrophone elements defining a hydrophone array.
(2) Description of the Prior Art
The gathering of data in an acoustic environment by means of an array of electrically interconnected hydrophone elements is of increasing importance, both for commercial and military purposes. Such arrays are usually linear and comprise a number of hydrophone elements distributed over a length in a linear array. In some applications, the arrays are two dimensional. In order for improved interpretation of information from any of these arrays it is important that the signals generated by each hydrophone element be in phase and preferably be within a given range of sensitivity. Consequently various means for testing hydrophone arrays have been developed with the following United States Letters Patent disclosing representative techniques and devices.
U.S. Pat. No. 4,205,394 to Pickens discloses a sealed cavity for containing a fluid, an acoustic projector, a reference hydrophone element and a hydrophone array to be tested. The reference hydrophone and the hydrophone array under test generate signals received by monitoring equipment responsive to the output of the acoustic projector. The monitoring equipment enables comparison between the responses of the reference hydrophone and the tested hydrophone array so as to evaluate the performance and polarity of the hydrophone array generally.
U.S. Pat. No. 4,223,397 to Bakewell, Jr. et al. discloses a device for use in a laboratory fluid tank that includes a tubular chamber through which a hydrophonic array extends. Fluid is directed through an inlet in the chamber to exit a port thereof. A turbulent flow is thereby attained as the fluid exits the chamber to enable monitoring of response of individual hydrophone elements to the turbulent flow.
U.S. Pat. No. 4,290,123 to Pickens discloses a device for travelling along a towed linear hydrophone array. The device includes an acoustic projector comprising a plurality of elements on a circular wall of a frame. The acoustic projector emits test signals as it moves along the towed array past individual hydrophone elements. A monitoring device connected to the hydrophone array provides information concerning the polarity at the center of an individual hydrophone element and other information used for calibration.
U.S. Pat. No. 4,320,468 to Montross discloses an apparatus and method for testing hydrophone elements mounted one to two feet apart in a linear array. The tester applies a repeatable low-level pressure pulse to individual hydrophone elements mounted in the array that constitutes a marine seismic streamer cable. More specifically, a solenoid when energized drives a plunger to impact the cable and produces a detectable pressure pulse. The hydrophone element at the solenoid generates a responsive signal. Successive signals from the individual elements due to moving the tester along the cable enable comparison of the amplitude, polarity and frequency of the signals.
U.S. Pat. No. 4,353,120 to Pickens discloses a pressure generator apparatus for insonifying selective portions of an elongated array of hydrophone elements. The apparatus comprises an elongated tube with reciprocating pistons disposed therein. Flexible caps at the ends of the tubes transmit the pressure waves generated by the pistons to a fluid medium in which the tube is maintained. The generator apparatus is preferably supported by a travelling cart, such as disclosed in the above-identified U.S. Pat. No. 4,290,123.
U.S. Pat. No. 4,375,679 to Park, Jr. et al. discloses a method and apparatus for testing hydrophone elements in a seismic streamer cable. The apparatus comprises an irregularly shaped chamber for being clamped to a portion of a seismic cable. A loudspeaker disposed on the chamber produces pressure waves that are directed into the chamber. The apparatus also includes a reference hydrophone element located in the chamber. Comparing signals from the hydrophone element under test and the reference hydrophone element enables an analysis of the polarity and sensitivity of the hydrophone element under test.
U.S. Pat. No. 5,210,718 to Bjelland et al. discloses a method for calibrating groups of hydrophone elements using a Helmholtz resonator. The hydrophonic groups or sections of a hydrophonic group are placed in the resonator's cavity and connected to a signal analyzer. The response of the hydrophonic groups or sections are compared to a reference hydrophone element within the cavity upon generation of pressure waves that strike the hydrophone elements. The output and the relative polarity of the hydrophone elements can be then analyzed by comparing the responses to those from the reference hydrophone element.
Thus, the prior art discloses a variety of apparatus and methods for calibration testing and other evaluations of hydrophone elements arranged to define a hydrophone array. However, many are not suited to manufacturing facilities. Many of the devices require testing in a fluid medium. Others require a reference hydrophone element; during testing the characteristics of such reference hydrophone elements can change with temperature, use and age. Still others of the references fail to account for generation of unwanted signals in other closely spaced hydrophone elements. That is, the references fail to provide a simple, easy to use device for readily determining the correct wiring of each individual hydrophone element in a hydrophone array.